Method of coating an electric discharge device

ABSTRACT

A METHOD OF MAKING AN ELECTRIC DISCHARGE DEVICE HAVING A VITREOUS COATING ON THE INTERIOR OF THE ENVELOPE IS DESCRIBED. THE VITREOUS COATING INHIBITS THE RELEASE OF OCCLUDED GASES FROM THE ENVELOPE WHICH UPON RELEASE WOULD ADVERSELY AFFECT THE OPERATION AND LIFE OF THE DEVICE.

W. T. MlLLlS May 18, 1971 METHOD OF COATING AN ELECTRIC DISCHARGE DEVICEOriginal Filed March 2, 1966 FIGJ.

FIGZ.

SAMPLE c SAMPLE A UNCOA TED ENVELOPE TUBE LIFE (HOURS) INVENTOR: WALTERT. MILLIS,

United States Patent Oihce 3,579,378 METHOD OF COATING AN ELECTRICDISCHARGE DEVICE Walter T. Millis, Owensboro, Ky., assignor to GeneralElectric Company Division of application Ser. No. 531,082, Mar. 2, 1966,

now Patent No. 3,465,196. Continuation-impart of application Ser. No.302,490, Aug. 9, 1963. This application Nov. 12, 1968, Ser. No. 798,830Int. Cl. C23d 5/04 US. Cl. 117-229 1 Claim ABSTRACT OF THE DISCLOSURE Amethod of making an electric discharge device having a vitreous coatingon the interior of the envelope is described. The vitreous coatinginhibits the release of occluded gases from the envelope which uponrelease would adversely affect the operation and life of the device.

This is a division of my application Ser. No. 531,082, filed Mar. 2,1966, now Pat. No. 3,465,196, entitled Electric Discharge Device WithMeans to Prevent Release of Occluded Gases From the Envelope Thereof andMethod; and a continuation-in-part of application Ser. No. 302,490,filed Aug. 9, 1963, and now abandoned, and assigned to the same assigneeas herein.

The present invention relates to electric discharge devices and moreparticularly to an improved electric discharge device with a vitreousenvelope having an insulative coating on the interior thereof and tomethods of preparing the same.

In electric discharge devices as heretofore known, the glass envelopesoften used are prepared by melting the components of the glasscomposition in the presence of the flame used to provide heat. Duringthe melting of the ingredients and the cooling of the resultant glasscomposition, combustion products such as Water vapor, carbon monoxide,and carbon dioxide, are in contact with the surface of the molten glassand a relatively large percentage of the combustion products becomeocculuded in the resultant glass. When such glass has been formed intoan envelope for an electrical device, the envelope is subject to heatingand to bombardment by electrons whereupon the combustion productsoccluded therein are released by electrical dissociation. For example,in vacuum tubes, the released gases reduce the degree of vacuum withinthe envelope and cause deterioration of cathode electron activity. Theseproducts react with the constituents of the cathode coating such asbarium, resulting in a lower initial cathode activity, and a gradualdrop in cathode activity throughout the life of the tube or a decideddrop in cathode activity early in its life with limited recoverythereafter.

This problem has long been recognized and many attempts have beenpreviously made to avoid the release of occluded gases. For example,carbon or metallic films have been coated on the inside of glassenvelopes and these have resulted in some improvement in the level andstability of cathode activity. However, in the case of carbon, thecoating flakes off when the structure of the device is inserted into theenvelope and such flakes, when present in the finished tube causeelectrical noise and intermittent shorts, thereby rendering the devicevirtually inoperative. Metallic films, sometimes flashed onto thesurface of the envelope during the evacuating process or subsequentlythereto, are less likely to produce such loose particles. However, theflashing is very diflicult to control and metal may deposit oninsulating areas between elements of the device producing inter-element3,579,378 Patented May 18, 1971 leakage, increasing inter-elementcapacitance or com pletely short circuiting the electrodes of thedevice.

The present invention is directed to an electric discharge device havinga new and improved vitreous envelope wherein the release of occludedgases is prevented, without encountering the above-described problems.

It is, accordingly, an object of the present invention to provide avitreous envelope for an electric discharge device wherein means areprovided to prevent the release of gases occluded in the envelope walls.

A further object of the present invention is the provision of a vitreousenvelope for an electric discharge device which is provided with ahigh-electrical resistance coating on its interior surface to preventthe release of occluded gases without increasing interelectrodecapacitance. The use of a high-electrical resistance coating also limitsthe amount of current which can flow in the glass, which current cancause evolution of gases therefrom.

Another object of the present invention is the provision of a method ofcoating the interior surface of a vitreous envelope for electricdischarge devices whereby the release of occluded gases is prevented.

A more specific object of the present invention is the provision of amethod of preparing a coating for the interior surface of vitreousenvelopes for electrical devices whereby the coating is formed withouthaving gases occluded therein.

Other objects and advantages of the present invention will becomeapparent as the description and illustration thereof proceeds.

Briefly, in accordance with one form of the present invention, avitreous coating is prepared by heating the constituents in anatmosphere free of combustion products or other gases susceptible ofocclusion. The vitreous coating is then converted to powdered form andapplied to the interior surface of a vitreous envelope, and a glaze orfrit is formed over substantially all areas of the interior surface fromwhich occluded gases might otherwise be released.

The electric discharge device is then completed by inserting a cagemember including a cathode having an emissive coating thereon,evacuating, and sealing, the result being a device having an envelopewith a barrier between occluded gases and the interior of the device.Alternatively, the entire envelope of the device may be prepared byheating in an atmosphere from which the combustion products of theheating means are excluded so that no occluded gases are present in theenvelope and the construction of the device proceeds without coating theinterior surface.

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself together with furtherobjects and advantages thereof may best be understood by reference tothe following description taken in connection with the appended drawingin which FIG. 1 is a longitudinal cross sectional view of an electricdischarge device constructed in accordance with my invention; and FIG. 2is a graph wherein data taken from vacuum tubes both with and withoutthe coating of the present invention are compared.

As shown in FIG. 1, an electric discharge device, according to thepresent invention, comprises a vitreous envelope, which includes atubular member 11 which may be cylindrical, as shown, or may be of anyother suitable geometrical configuration, and, as will appear, isfabricated of lead or lime glass as desired or required. The envelope 10is completed by a stem or base portion 12 which closes the open end ofthe member 11. The major portion of the internal surface of the member11 is coated with a tightly adhering layer of glaze 13 ofhigh-electrical resistance material which functions to entrap, or sealoff, any gases or vapors that may be adhering to or occluded in thewalls of the member 11.

The electric discharge device further includes a cage member 15 whichmay be of any suitable electrical configuration such as a rectifier,triode, pentode, etc. As depicted, the cage member includes a tubularanode 17, a tubular cathode 19 and wound grids 20 and 21 supportedbetween a pair of insulating supports 22 and 23, thereby providing atetrode structure. The cathode 19 is provided with an electron emissivecoating 24 of, for example, barium oxide which emits electrons whenheated by a heater filament (not shown) normally provided within thecathode.

The method and envelope of the present invention are best described inthree parts: First, the method of preparing a coating; second, themethod of applying a coating; and third, the method of manufacture ofthe electric discharge device.

In prcparing a vitreous coating according to one aspect of the presentinvention, one can place either a prepared composition having thedesired ingredients or the separate ingredients themselves in anenclosed furnace having an external heating means. Although thecomposition of the coating is not critical to the present invention, thefollowing considerations are important. First, the coefiicient ofexpansion of the coating should be adequately matched with that of thevitreous envelope so that the coating will not break away from theenvelope or cause cracking of the envelope. Secondly, the melting pointof the coating should be lower than that of the envelope so that thecoating can be glazed over the interior surface, as will be hereinafterdescribed, without melting the envelope. Both of these factors arecontrolled by the composition of the coating and, therefore, it is ofpractical importance to select appropriate ingredients. For example, ithas been found that coatings comprising from about 60% to about 80% leadmonoxide are appropriate for use in conjunction with lime glass which isthe most common material presently used for vitreous envelopes ofelectrical devices. It has also been found that glasses containingapproximately 50% silica are also appropriate.

As an added precaution, and to further ensure removal of water vapor,one may heat the mixture in vacuo to a temperature appropriate to removesuch occluded water vapor and thereafter providing a dry air atmospherein the furnace and continuing the heating to the melting point of theingredients or of the composition. If the initial heating is notperformed in a vacuum it has been found that the pressure of theatmosphere may not permit removal of all of the water vapor in themixture. If I the vacuum is maintained throughout the heating process,some components of the coating would be released as vapors, thuschanging the composition of the coating. However, it is contemplated,according to the present invention, to employ both vacuum and airheating throughout since either procedure will result in the exclusionof substantially all of the combustion product gases and this willprovide the benefits of this method of preparation.

An important feature of the present invention is embodied in theabove-described preparation of the coating, namely, that the coating canbe prepared in such a manner as to exclude therefrom combustion productsof the flame heretofore used to melt glasses used for vitreousenvelopes. The absence of these gases enables operation of the tube madewith an envelope according to this invention at higher levels and forlonger periods because the gases cannot be released to cause thedifiiculties previously described.

The method of applying the coating to the interior surface of thevitreous envelope will now be described. The material which will formthe coating is ground into a fine powder and is mixed with anappropriate suspension medium such as alcohol or ethanol, which functionto hold the powder on the surface of the envelope while the glazing isperformed. The mixture containing the powdered coating material ispoured into the interior of the vitreous envelope so as to cover alldesired areas of the interior surface and excess solution is removed asby pouring off. The coated envelope is then heated to any suitabletemperature above the melting point of the coating material and belowthat of the envelope material and preferable to the annealingtemperature of the latter, to remove the binder by drying and to glazethe powder to the interior surface of the envelope. The glazing may thusbe performed at the same time as the annealing of the envelope.

As specific examples of the vitreous coatings used, glasses having thefollowing compositions were found to be appropriate for use with lime orlead glass envelopes and, when prepared in accordance with theabovedescribed steps, the electrical device life and cathode activitywere greatly increased:

The above vitreous coatings were prepared in accordance with thepreferred embodiment of the invention, by placing a prepared glasshaving the appropriate composition in an enclosed furnace havingexternal heating means. The material is then heated in vacuo to atemperature appropriate to remove occluded water vapor. A dry airatmosphere is then introduced into the system and the heating continuedto the melting point of the glass to remove occluded gases and provide avitreous coating material substantially free of all of combustionproducts. This material is then ground to a fine powder and mixed withalcohol to form a suspension. 'Ihis suspension is poured into theinterior of the vitreous envelope of the discharge device and the excesspoured off after the desired areas have been covered by the suspension.The envelope covered with the suspension is then heated to a temperatureabove the melting point of the coating material and below that of theenvelope to glaze the powder to the interior of the envelope.

A cage member 15 including a cathode 19 having an emissive coating 24thereon is secured upon a base member 12. The assembled cage and baseare juxtapositioned with the envelope 10 with the cage in the positiondepicted in FIG. 1.

The base member 12 is then sealed to the bottom of the tubular member 11to complete the envelope and enclose the cage member 15. The electricdischarge device is completed by exhausting the envelope through anexhaust tubulation (now shown), the exhaust tubulation then being sealedoff.

The resultant article comprises the vitreous envelope 11 having acoating 13 on the interior surface thereof, as shown in FIG. 1. Thecoating is a glaze which covers all areas of the interior surface whichmay be exposed to the evacuated region of the electrical device so thatoccluded gases in the vitreous envelope cannot be released to theevacuated area by operational heating or by electron bombardment. Thecoating itself is prepared so that it is free of such occluded gasesand, therefore, the evacuated space will remain free of such occludedgases throughout the life of the device.

The graph of FIG. 2 illustrates the advantages gained in vacuum tubecathrode activity resulting from coating the envelope in accordance withthis aspect of the present invention. The graph is a plot of an emissionfactor or of the cathode or transconductance of the device during thelife of the device for three otherwise identical vacuum tube devices. Itwill be noted that the hours of life scale is not linear, the pointsbeing plotted at regular intervals regardless of the actual intervalsbetween measurements. Since the purpose of this graph is to indicate therelative operation with and without envelope coating, the distancebetween plotted measurements are not important so long as they are thesame for each tube. The actual hours between measurements are indicatedby the abscissas.

It will be seen from this graph that tubes coated with Samples A and Bmaintained a much higher transconductance throughout their usable lifethan did the uncoated tube. The higher level of cathode activity,indicated by the higher transconductance, contributes significantly toimprove tube operation. Furthermore, since the coating is insulativerather than conductive, there is no problem of undesirably changing theinter-element capacitance nor of short circuiting the elements, as hasbeen experienced with conductive coatings. The insulative glaze of theinvention has been found to be highly adherent to the vitreous envelopeand there is no problem of flaking of particles on insertion of the tubeelements which may cause electrical noise of interference. Since thecoating is performed before insertion of the elements, the locating ofthe coating can be carefully controlled as desired without encounteringthe difficulty of positioning a flashed coating or other coating appliedafter the completion of the tube or device.

In the tubes actually used for these measurements illustrated, only 75%of the interior was coated for convenience. -In practice, the entireinner surface exposed to the tube elements may be coated as far asnecessary to result in further improvement due to the exclusion ofoccluded gases from the electron bombarded portions of the en elope.

An alternative method of preparing an electrical device envelope havinga vitreous coating is provided by my invention by using a silicone resindissolved in a volatile solvent such as naphtha or toluene. Siliconeresins, for example, a methyl methoxypolysiloxane made by the process ofExample 1 of US. Pat. 'No. 2,810,704, of Karl W. Krantz, patented Oct.22, 1957 and assigned to the assignee of this application, has beenfound to be a satisfactory coating preparation when diluted to a solidconcentration with petroleum spirits. The solution is poured into theenvelope, the excess is poured off and the adhering portion is dried.The envelope is fired, for example during annealing, at a temperaturesuch as 400 C.- 450 C. During firing, the resin breaks down chemicallyto form a vitreous coating or glaze which comprises silicon monoxide andsilicon dioxide. This vitreous coating per forms the same function asthe coatings described hereinabove, namely, to prevent the release ofoccluded gases. The resultant coatings in each case are vitreous and aregood electrical insulators.

A further method of obtaining electric discharge devices having improvedtransconductance characteristics is provided by my invention by using anamorphous silica which is essentially pure SiO in a non-crystallineform. Such material is obtained as a by-product, actually a wasteproduct, in the production of silicon metal. During such production acertain amount of silicon vaporizes and is carried through the aircirculation system. To prevent discharge of this waste into theatmosphere, it is collected in the form of the aforementioned amorphousquartz. Such material has been found to be highly suitable for themanufacture of electric discharge devices in accordance with theinvention, not only because of economy, but also because of ease inprocessing. Because of the nature of the material, i.e., its lack ofcrystallinity and its small particle size, it has been found to readilyadhere to the interior of the envelope Without firing, however, firingimproves the adherence further, and being a stable compound will notdecompose and add contaminants to the tube during operation thereof.This material has a chemical analysis of:

Sample C: Percent 5 F6203 .34 MnO .09

CaO .35

MgO .23

' K 0 .59 Na O .07

S0 .35 C+H O+CO 1.68

Si0 Balance Thus, in accordance with this aspect of the invention, aquantity of Sample C amorphous silica having a grain size betweenapproximately .25 and .02 micron was mixed with methanol to form a thinslurry. The mathanol was used since it evaporates readily withoutleaving a hydrocarbon residue. This slurry was poured into a tubularmember 11 and the excess poured out leaving a film of the slurry on theinterior walls thereof. The methanol readily evaporates leaving anessentially pure Si0 vitreous coating on the envelope. The evaporationof methanol may be conveniently speeded up by heating to approximately100 C. This coating is very adherent and does not flake off on shakingor other handling of the bulb.

Before sealing this tubular member with an assembled cage and basemember a portion of the interior wall adjacent the areas to be sealed tothe base is wiped clean. The tube is completed in the same manner asabove.

As shown in the graph of FIG. 3, the curve for Sample C illustratesagain the advantages gained in electric discharge devices resulting fromcoating the envelope in accordance with this further aspect of theinvention. It will be noted that Sample C, the amorphous silica, in thiscase, does not exhibit the initial slump exhibited by Samples A and Band to a larger extent the uncoated envelope.

While the latter examples have been disclosed as being placed on theenvelope by a pouring method it will be appreciated that painting,spraying or other convenient coating technique may be employed. Also,while the im- 45 provement of the transconductance characteristic of thetube has been specifically disclosed, it will be appreciated that theinvention will be useful for maintaining other electricalcharacteristics of the tube, as for example, interelectrode capacitance.

It is to be noted that the entire envelope material could, if desired,be made from a composition free of occluded gases, thus obviating theneed for the coating for entrapment of such gases. However, in view ofthe mass produc- 5 tion of vitreous envelopes for electrical devices,and the practicality of flame-melting large quantities of glass to beused for this purpose, the more economical method is to apply thecoating of this invention to the interior of an envelope.

The specific embodiments, methods and compositions described herein arepresented merely as examples of the many forms the practice of thisinvention may take. Therefore, it is intended in the appended claim tocover all modifications and variations which come within the true spiritand scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A method of improving the transconductance of an electric dischargedevice including a coated cathode in a vitreous envelope by preventingdischarge of occluded gases from said envelope to the interior of saiddevice during operation comprising the steps of:

(a) coating the interior of said envelope with an insula- -tive coatingof lead glass, said glass being prepared by 7 8 heating the componentsthereof in a vacuum until References Cited water vapor therein has beenremoved and thereafter providing a dry air atmosphere, said coatingcomposition being substantially free of occluded giggi z f 7 gases; 52,810,660 10/1957 Carpenter 11797X (b) heating said components in saiddry air atmosphere to the melting point thereof to form a vitreous com-WILLIAM L. JARVIS, Primary Examiner position; and

(c) sealing a cage member of said discharge device in Us CL saidenvelope. 117-97; 313 223,197; 65-60

